Chemically modified titania hydrolysates: Physical properties

Titania hydrolysates prepared from the hydrolysis of unmodified tetraisopropoxytitanium(IV) and tetraisopropoxytitanium(IV) modified before hydrolysis by replacement of the tetraisopropoxy groups via a reaction with carboxylic acids, including acetic, propanoic, and butanoic acid, are shown by FT-Raman and X-ray diffraction to be amorphous with no clear long-range ordering. The hydrolysate spectra show three broad bands around 210, 440, and 570 cm(-1), which do not correspond with any known titania phase. In the modified systems, nucleophilic substitution and elimination of alkoxy groups is virtually complete. Charge separation induced by modifying ligands ensures that the removal of carboxylates is incomplete, although not occurring to the same degree in the three systems. Elimination of carboxylic acid groups is increasingly favorable with longer chain length, but the modifying groups also become more difficult to protonate. The hydrophobicity of the carboxylate groups increases with chain length, and this may also reduce the rate of nucleophilic attack by water upon carboxylate ligands. The net effect of the charge distribution and the hydrophobicity within the modified species is for the quantities of residual carboxylate to increase with the chain length of the modifying ligands. Modification also influences the particle size, the surface area, and the porosity of the hydrolysate aggregates. The rate and extent of polycondensation decreases with carboxylic acid modification and the chain length of the acid. The specific surface areas of the modified hydrolysates are reduced by the presence of residual carboxylate groups. Unreacted carboxylate groups have a significant influence upon the crystallization and phase-transformation properties of the hydrolysates. In the acetate- and propanoate-modified hydrolysates, unreacted carboxylate inhibits crystallization and phase transformation of the solids, elevating the temperatures at which these processes occur. In the butanoate-modified hydrolysates, carboxylate ligands are found in larger quantities, enhancing these processes and decreasing the crystallization and phase transformation temperatures, to the extent that the anatase --> rutile phase transformation occurs at lower temperatures in butyric hydrolysates than in unmodified materials.